Frusto-conical spinal implant

ABSTRACT

The present invention is directed to a variety of interbody spinal fusion implants having at least a partially frusto-conical configuration. The spinal fusion implants of the present invention may be relatively solid or hollow and may have surface roughenings to promote bone ingrowth and stability. The spinal fusion implants of the present invention may have wells extending into the material of the implant from the surface for the purpose of holding fusion promoting materials and to provide for areas of bone ingrowth fixation. A variety of surface irregularities may be employed to increase implant stability and implant surface area, and/or for the purpose of advancing the spinal fusion implant into the fusion site.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/085,706,filed Mar. 19, 2005, now U.S. Pat. No. 7,691,148; which is acontinuation of application Ser. No. 08/484,928, filed Jun. 7, 1995, nowU.S. Pat. No. 6,923,810; all of which are incorporated herein byreference.

The disclosures of U.S. Pat. Nos. 5,015,247; 5,484,437; and 6,080,155are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to interbody spinal fusionimplants, and in particular to spinal fusion implants configured torestore and maintain two adjacent vertebrae of the spine in anatomicallordosis.

2. Description of the Related Art

Interbody spinal fusion refers to the method of achieving bony bridgingbetween adjacent vertebrae through the disc space, the space betweenadjacent vertebrae normally occupied by a spinal disc. Numerous implantsto facilitate such a fusion have been described by Cloward, Brantigan,and others, and are known to those skilled in the art. Generally,cylindrical implants offer the advantage of conforming to an easilyprepared recipient bore spanning the disc space and penetrating intoeach of the adjacent vertebrae. Such a bore may be created by use of adrill. It is an anatomical fact that both the cervical spine and thelumbar spine are normally lordotic, that is convex forward. Suchalignment is important to the proper functioning of the spine. Commonly,those conditions which require treatment by spinal fusion are associatedwith a loss of lordosis.

Therefore, there exists a need for spinal fusion implants that permitfor the restoration of anatomical lordosis.

SUMMARY OF THE INVENTION

The present invention is directed to a variety of interbody spinalfusion implants having at least a partially frusto-conical configurationto achieve a desired anatomical lordosis of the spine. In the preferredembodiment, the spinal fusion implants of the present invention have anouter locus in which at least some of the points of the implant comprisea partially or fully frusto-conical shape substantially along thoseportions of the implant in contact with the adjacent vertebrae of thespine and have an insertion end and a trailing end. The spinal fusionimplants of the present invention may be further modified so that whilethe upper and lower surfaces are portions of a frusto-cone, at least oneside portion may be truncated to form a planar surface that is parallelto the central longitudinal axis of the implant to form straight walls.These implants may have a more tapered aspect at the insertion end ofthe implant to facilitate insertion. The spinal fusion implants of thepresent invention may be relatively solid and/or porous and/or hollow,and may have surface roughenings to promote bone ingrowth and stability.

The spinal fusion implants of the present invention may have wellsextending into the material of the implant from the surface for thepurpose of holding fusion promoting materials and to provide for areasof bone ingrowth fixation. These wells, or holes, may pass either intoor through the implant and may or may not intersect. The spinal fusionimplants of the present invention may have at least one chamber whichmay be in communication through at least one opening to the surface ofthe implant. Said chamber may have at least one access opening forloading the chamber with fusion promoting substances. The access openingmay be capable of being closed with a cap or similar means. Stillfurther, a variety of surface irregularities may be employed to increaseimplant stability and implant surface area, and/or for the purpose ofallowing the spinal fusion implant to be inserted easily but to resistmotion in the oppostion direction. The exterior of the spinal fusionimplant of the present invention may have wholly or in part, a roughfinish, knurling, forward facing ratchetings or other surfaceirregularities sufficient to achieve the purpose described.

The spinal fusion implants of the present invention offer significantadvantages over the prior art implants:

1. Because the spinal fusion implants of the present invention are atleast partially frusto-conical in shape, those that taper from theleading edge to the trailing edge are easy to introduce and easy tofully insert into the spinal segment to be fused.

2. The shape of the implants of the present invention is consistent withthe shape of the disc, which the implants at least in part replace,wherein the front of the disc is normally taller than the back of thedisc, which allows for normal lordosis. The implants of the presentinvention are similarly taller anteriorly than they are posteriorly.

3. The spinal fusion implants of the present invention conform to ageometric shape, which shape is readily producible at the site offusion, to receive said spinal fusion implants.

The spinal fusion implants of the present invention can be made of anymaterial appropriate for human implantation and having the mechanicalproperties sufficient to be utilized for the intended purpose of spinalfusion, including various metals such as cobalt chrome, stainless steelor titanium including its alloys, various plastics including those whichare bio-absorbable, and various ceramics or combination sufficient forthe intended purpose. Further, the spinal fusion implants of the presentinvention may be made of a solid material, a mesh-like material, aporous material and/or may comprise, wholly or in part, materialscapable of directly participating in the spinal fusion process, or beloaded with, composed of, treated or coated with chemical substancessuch as bone, morphogenic proteins, hydroxyapatite in any of its forms,and osteogenic proteins, to make them bioactive for the purpose ofstimulating spinal fusion. The implants of the present invention may bewholly or in part bioabsorbable.

OBJECTS OF THE PRESENT INVENTION

It is an object of the present invention to provide a spinal fusionimplant that is easily inserted into the spine, having a tapered leadingend;

It is another object of the present invention to provide a spinal fusionimplant that tapers in height from one end to the other consistent withthe taper of a normal spinal disc;

It is yet another object of the present invention to provide a spinalfusion implant that is capable of maintaining anatomic alignment andlordosis of two adjacent vertebrae during the spinal fusion process;

It is still another object of the present invention to provide a spinalfusion implant that is self stabilizing within the spine;

It is yet another object of the present invention to provide a spinalfusion implant that is capable of providing stability between adjacentvertebrae when inserted;

It is still another object of the present invention to provide a spinalfusion implant that is capable of participating in the fusion process bycontaining, being composed of, or being treated with fusion promotingsubstances;

It is further another object of the present invention to provide aspinal fusion implant that is capable of spacing apart and supportingadjacent vertebrae during the spinal fusion process;

It is still further another object of the present invention to provide aspinal fusion implant that is consistent in use with the preservation ofa uniform thickness of the subchondral vertebral bone;

It is another object of the present invention to provide a spinal fusionimplant having a shape which conforms to an easily producedcomplementary bore at the fusion site; and

It is a further object of the present invention to provide afrusto-conical spinal fusion implant which may be placed side by sideadjacent to a second identical implant across the same disc space, suchthat the combined width of the two implants is less than sum of theindividual heights of each implant.

These and other objects of the present invention will become apparentfrom a review of the accompanying drawings and the detailed descriptionof the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of an embodiment of the spinal fusionimplant of the present invention having a frusto-conical body and asurface configuration comprising ratchetings for engaging bone, withwells and channels for bone ingrowth.

FIG. 1A is an enlarged fragmentary view along line 1A of FIG. 1illustrating the surface configuration of the implant of FIG. 1.

FIG. 2 is a cross sectional view along line 2-2 of the implant of FIG. 1illustrating the channels and wells of the implant of the presentinvention.

FIG. 3 is a cross sectional view along line 3-3 of the implant of FIG. 1illustrating the channels and wells of the implant of the presentinvention.

FIG. 3A is a side elevational view of an alternative embodiment of thespinal fusion implant of the present invention having a frusto-conicalbody and a plurality of ratchetings forming a cylindrical externalconfiguration.

FIG. 4 is a side elevational view of an alternative embodiment of thespinal fusion implant of the present invention having truncated sidesforming a planar surface parallel to the longitudinal axis of theimplant and ratchetings having a radius and height that are notconstant.

FIG. 5 is a top plan view of the spinal fusion implant shown in FIG. 4.

FIG. 6 is a side elevational view of an alternative embodiment of thespinal fusion implant of the present invention having a body that ismade out of a fibrous mesh-like material that is partiallyfrusto-conical with one side that is truncated shown next to anidentical second implant illustrated in hidden line.

FIG. 7 is sectional view along line 7-7 of the implants of FIG. 6.

FIG. 8 is an enlarged fragmentary view along line 8 of FIG. 6illustrating the surface configuration of the implant of FIG. 6.

FIG. 9 is an enlarged fragmentary view along line 8 of FIG. 6illustrating an alternative embodiment of the surface configuration ofthe implant of the present invention made of a cancellous material.

FIG. 10 is a cross sectional view along lines 10-10 of FIG. 9illustrating the alternative embodiment of the surface configuration ofthe implant of the present invention made of a cancellous material.

FIG. 11 is a side elevational view in partial cut-away of an alternativeembodiment of the spinal fusion implant of the present invention havinga body that is frusto-conical and a surface configuration comprising aplurality of spaced apart posts.

FIG. 12 is an enlarged fragmentary sectional view along lines 12-12 of

FIG. 11 illustrating the surface configuration of the implant of FIG.11.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, an embodiment of the spinal fusion implant of thepresent invention is shown and generally referred to by the numeral 220.The implant 220 has a frusto-conical body 222 and an outer locus that isgenerally frusto-conical substantially along the portion of the implant220 that is in contact with the adjacent vertebrae of the spine. Theimplant 220 has a surface configuration of forward facing ratchetings240 suitable for engaging the bone of the adjacent vertebrae. Each ofthe plurality of ratchetings 240 has a bone engaging edge 242 and rampedportion 244. The ratchetings 240 have a radius R₄ measured from thecentral longitudinal axis L₄ of the implant 220 that increases from theinsertion end 224 to the trailing end 226. The height of the ratchetings240 measured from the body 222 is constant throughout the length ofimplant 220.

The orientation of the ratchetings 240 makes the insertion of theimplant 220 easier than its removal, as the ramped portions 244 act asan inclined plane on the way in, while the bone engaging edges 242resist motion in the opposite directions. These forward facingratchetings 240 tend to urge the implant 220 forward until the unremovedbone of the vertebrae blocks further motion resulting in a very stablespine and implant construct.

The implant 220 has a recessed slot 234 at its trailing end 226 forreceiving and engaging insertion instrumentation for inserting theimplant 220. The recessed slot 234 has a threaded opening 236 forthreadably attaching the implant 220 to instrumentation used forinserting the implant 220.

In the preferred embodiment, the bone engaging edges 242 of theratchetings 240 have a height at a highest point measured from the body222 (root diameter) of the implant 220 in the range of 0.25-2.0 mm, withthe preferred height being 0.4 mm for use in the cervical spine and 1.25mm for use in the lumbar spine.

Referring to FIGS. 2 and 3, cross sectional views of implant 220 areshown. The implant 220 has channels 250 passing through the implant 220and wells 260 formed in the surface of the implant 220. The wells 260may or may not communicate with the channels 250. In the preferredembodiment of implant 220, the channels 250 have a diameter in the rangeof 0.1 mm to 6 mm, with 2-3 mm being the preferred diameter. The wells260 have a diameter in the range of 0.1 mm to 6 mm, with 1-3 mm beingthe preferred diameter range. It is appreciated that although thechannels 250 and wells 260 are shown having a generally roundedconfiguration, it is within the scope of the present invention that thechannels 250 and wells 260 may have any size, shape, configuration, anddistribution suitable for the intended purpose.

Referring to FIG. 1A, the implant 220 has an outer surface 238 that isporous to present an irregular surface to the bone to promote boneingrowth. The outer surface 238 is also able to hold fusion promotingmaterials and provides for an increased surface area to engage the bonein the fusion process and to provide further stability. It isappreciated that the outer surface 238, and/or the entire implant 220,may comprise any other porous material or roughened surface sufficientto hold fusion promoting substances and/or allow for bone ingrowthand/or engage the bone during the fusion process. The implant 220 may befurther coated with bioactive fusion promoting substances including, butnot limited to, hydroxyapatite compounds, osteogenic proteins and bonemorphogenic proteins. The implant 220 is shown as being solid, howeverit is appreciated that it can be made to be substantially hollow orhollow in part.

While the implant 220 is shown as being solid, it is appreciated thatthe implant 220 can be hollow at least in part to provide an internalchamber for holding bone or any fusion promoting material. Such animplant could have openings to allow bone external to the implant togrow into the internal chamber. Such structure is disclosed in detail inapplication Ser. No. 08/390,131, now U.S. Pat. No. 5,593,409, andapplication Ser. No. 08/074,781, now U.S. Pat. No. 5,484,437, both ofwhich are incorporated herein by reference.

Referring to FIG. 3A, an alternative embodiment of the implant 220 isshown and generally referred to by the numeral 220′. The implant 220′ issimilar in configuration to implant 220 except that the body 222′ of theimplant is frusto-conical in configuration and the ratchetings 240′ havea radius R₃ measured from the longitudinal central axis L₄ that isconstant in size from the insertion end 224′ to the trailing end 226′.The ratchetings 240′ each have a height measured from the body 222′ thatis not constant throughout the length of the implant 220′ and decreasesfrom the insertion end 224′ to the trailing end 226′. In this manner,the ratchetings 240′ form an external configuration of the implant 220′that is substantially cylindrical in shape, while the body 222′ isfrusto-conical. The insertion end of implant 220′ may have a taperedportion 223′ of lesser diameter to facilitate insertion of the implant220′. The insertion end of the implant may also be larger than thetrailing end where so desired.

Referring to FIGS. 4 and 5, an alternative embodiment of the implant 220is shown and generally referred to by the numeral 220″. The implant 220″is similar in configuration to implant 220 and has ratchetings 240″having a radius R₅ measured from the longitudinal central axis L₅ thatincreases in size from the insertion end 224″ to the trailing end 226″.The ratchetings 240″ each have a height measured from the body 222″ thatis not constant throughout the length of the implant 220″. In thepreferred embodiment, the ratchet radius R₅ and the ratchet heightincrease in size from the insertion end 224″ to the trailing end 226″.

As shown in FIG. 5, the implant 220″ has truncated sides 270 and 272forming two planar surfaces which are diametrically opposite and areparallel to the longitudinal axis L₅. In this manner, two implants 220″may be placed side by side with one of the sides 270 or 272 of eachimplant touching, such that the area of contact with the bone of theadjacent vertebrae and the ratchetings 240″ is maximized. Alternatively,the implant 220″ may have one truncated side.

Referring to FIGS. 6-8, an alternative embodiment of the spinal fusionimplant of the present invention is shown and generally referred to bythe numeral 320 a. The implant 320 a is shown placed next to a secondidentical implant 320 b shown in hidden line. The implant 320 a has abody 322 that is made of a mesh-like material comprising strands, whichmay be made of metal, that are pressed together and molded into apartially frusto-conical configuration substantially along the portionof the implant 320 a in contact with the adjacent vertebrae of thespine. The implant 320 a has an insertion end 324 and a trailing end 326and may be made wholly or in part of a solid material and/or a porousmaterial, and/or a mesh-like material. The implant 320 a may have asurface comprising of a porous material, a mesh-like material, or have asurface that is roughened. It is appreciated that the implant 320 a maybe solid or may be partially hollow and include at least one internalchamber. As shown in FIG. 8, the mesh-like material comprises strandsthat are formed and pressed together such that interstices 339, capableof retaining fusion promoting material and for allowing for boneingrowth, are present between the strands in at least the outer surface338 of implant 320 a.

Referring to FIGS. 9 and 10, alternatively the implant may be made of acancellous material 350, similar in configuration to human cancellousbone, having interstices 352 such that the outer surface has aconfiguration as shown in FIGS. 9 and 10. As the implant may be madeentirely or in part of the cancellous material 350, the interstices 352may be present in the outer surface and/or within the entire implant topromote bone ingrowth and hold bone fusion promoting materials.

Referring again to FIG. 7, the implant 320 a is partiallyfrusto-conical, similar in shape to implant 220 but having at least onetruncated side 340 that forms a planar surface parallel to the centrallongitudinal axis of implant 320 a. The truncated side 340 allows forthe placement of two implants 320 a and 320 b closer together whenplaced side by side between two adjacent vertebrae as set forth in U.S.patent application Ser. No. 08/390,131, now U.S. Pat. No. 5,593,409,incorporated herein by reference. Implant 320 a may be partiallythreaded or may otherwise resemble any of the other embodiments hereindescribed or that are functionally equivalent.

Referring to FIG. 11, a side elevational view in partial cut-away of analternative embodiment of the implant of the present invention is shownand generally referred to by the numeral 420. The implant 420 has a body422 that is frusto-conical in shape substantially along the portion ofthe implant 420 that is in contact with the adjacent vertebrae of thespine and has an insertion end 424 and a trailing end 426. The implant420 has an outer surface 438 that is capable of receiving and holdingbone, or other materials capable of participating in the fusion processand/or capable of promoting bone ingrowth. In the preferred embodiment,the surface 438 comprises a plurality of posts 440 that are spaced apartto provide a plurality of interstices 442 which are partial wells withincomplete walls capable of holding and retaining milled bone materialor any artificial bone ingrowth promoting material. The implant 420 maybe prepared for implantation by grouting or otherwise coating thesurface 438 with the appropriate fusion promoting substances.

Referring to FIG. 12, an enlarged view of the surface 438 of implant 420is shown. In the preferred embodiment, the posts 440 have a head portion444 of a larger diameter than the remainder of the posts 440, and eachof the interstices 442 is the reverse configuration of the posts 444,having a bottom 446 that is wider than the entrance 448 to theinterstices 442. Such a configuration of the posts 440 and interstices442 aids in the retention of bone material in the surface 438 of theimplant 420 and further assists in the locking of the implant 420 intothe bone fusion mass created from the bone ingrowth. As the boneingrowth at the bottom 446 of the interstices 442 is wider than theentrance 448, the bone ingrowth cannot exit from the entrance 448 and islocked within the interstice 442. The surface 438 of the implant 420provides for an improvement in the available amount of surface areawhich may be still further increased by rough finishing, flocking orotherwise producing a non-smooth surface.

In the preferred embodiment, the posts 440 have a maximum diameter inthe range of approximately 0.1-2 mm and a height of approximately 0.1-2mm and are spaced apart a distance of approximately 0.1-2 mm such thatthe interstices 442 have a width in the range of approximately 0.1 to 2mm. The post sizes, shapes, and distributions may be varied within thesame implant.

While the present invention has been described in detail with regards tothe preferred embodiments, it is appreciated that other variations ofthe present invention may be devised which do not depart from theinventive concept of the present invention. In particular, it isappreciated that the various teachings described in regards to thespecific embodiments herein may be combined in a variety of ways suchthat the features are not limited to the specific embodiments describedabove.

Each of the features disclosed in the various embodiments and theirfunctional equivalents may be combined in any combination sufficient toachieve the purposes of the present invention as described herein.

1. An interbody spinal fusion implant for insertion across the surgically corrected height of a disc space between two adjacent vertebral bodies of a human spine, said implant comprising: a leading end for insertion first into the disc space, a trailing end opposite said leading end, a length between said ends, and a mid-longitudinal axis passing through said leading and trailing ends, said leading end including an exterior surface being arcuate in both a first plane and a second plane adjacent the intersection of the mid-longitudinal axis therewith, said first plane and said second plane extending along the mid-longitudinal axis and being perpendicular to one another; opposed upper and lower portions adapted to contact the adjacent vertebral bodies when implanted in the spine, said opposed upper and lower portions of said implant including at least one opening in communication with one another for permitting for the growth of bone from vertebral body to vertebral body through said implant; and non-threaded bone engaging projections for engaging said implant to the adjacent vertebral bodies, said bone engaging projections extending from at least a portion of said opposed upper and lower portions, at least two of said bone engaging projections being formed as a frusto-conical segment, each of said frusto-conical segments having a forward-facing facet directed at least in part toward said leading end, each of said forward-facing facets having a slope, the slope of at least one of said forward-facing facets being different from the slope of another of said forward-facing facets of said frusto-conical segments.
 2. The implant of claim 1, wherein said implant includes two opposed sides between said upper and lower portions, at least one of said sides being planar along at least a portion of the length.
 3. The implant of claim 1, wherein said upper and lower portions form at least a portion of a cylinder.
 4. The implant of claim 1, wherein said bone engaging projections include a plurality of ratchetings.
 5. The implant of claim 1, wherein said implant has a hollow interior and an access opening for accessing said hollow interior.
 6. The implant of claim 5, further comprising a cap for closing said access opening.
 7. The implant of claim 1, wherein said trailing end is adapted to engage instrumentation for the insertion of said implant.
 8. The implant of claim 1, wherein said implant is configured to be placed in close proximity in a side by side alignment to a second implant, said first and second implants when placed together having a combined overall width that is less than the sum of the individual maximum diameters of each of said first and second implants.
 9. The implant of claim 1, wherein said upper and lower portions are at least in part arcuate.
 10. The implant of claim 1, wherein said upper and lower portions are at least in part arcuate from side to side along at least a portion of the length of said implant.
 11. The implant of claim 1, wherein said upper and lower portions are angled relative to one another so as to maintain an angulation of the adjacent vertebral bodies relative to one another.
 12. The implant of claim 11, wherein said upper and lower portions are at least in part arcuate.
 13. The implant of claim 1, wherein said upper and lower portions each include a portion that is an arc of the same circle.
 14. The implant of claim 1, wherein each of said frusto-conical segments have a rearward-facing portion directed at least in part toward said trailing end and extending transversely to the mid-longitudinal axis.
 15. The implant of claim 1, wherein each of said frusto-conical segments have a rearward-facing portion directed at least in part toward said trailing end, said rearward-facing portions having the same slope.
 16. The implant of claim 1, wherein said frusto-conical segments are positioned one in front of another along the length of said implant.
 17. The implant of claim 1, wherein said frusto-conical segments are immediately adjacent one another along the length of said implant.
 18. The implant of claim 1, wherein the slope of said forward-facing facets of said frusto-conical segments decreases from one of said ends toward another of said ends.
 19. The implant of claim 1, wherein the slope of said forward-facing facets of said frusto-conical segments decreases from said leading end toward said trailing end. 